Caching with Rails: An overview

Ruby on Rails Guides: Caching with Rails: An overview

One of the issues with Page Caching is that you cannot use it for pages that require to restrict access somehow. This is where Action Caching comes in. Action Caching works like Page Caching except for the fact that the incoming web request does go from the webserver to the Rails stack and Action Pack so that before filters can be run on it before the cache is served. This allows authentication and other restriction to be run while still serving the result of the output from a cached copy.

Full article: http://guides.rubyonrails.org/caching_with_rails.html